This disclosure is directed to a method of making a bearing assembly and also describes a bearing assembly with an insert for that bearing assembly whereby improved performance can be obtained. It is particularly intended for use in rotating devices such as turbine motors and the like which are exposed to drilling fluids in a well borehole.
In a typical downhole device operated at the lower end of a drill string, a flow of several hundred gallons of drilling fluid per minute is typically pumped down a drill string to lubricate the equipment at the bottom of the drill string and to wash cuttings away from the area where the drilling fluid returns to the surface. This upward flow is in the annular space on the exterior of the drill string. Ordinarily, drilling is occasioned by the use of a three cone drill bit that is mounted on the drill string which is rotated from the surface. In recent years, special drilling circumstances have enhanced the necessity of drilling a well borehole through the use of downhole motors and the like. This requires rotating equipment to be affixed to the drill string which is mud powered so that the rotating equipment must operate in this environment completely surrounded by the drilling fluid. Drilling fluid is typically formed of various earth materials such as clay and may include heavier particles such as barites. It is intended to lubricate the surfaces. Inevitably, the material is an abrasive even though the particles might be relatively small.
The drilling fluid is therefore circulated down the drill string and flows all around the rotating equipment. The present disclosure is directed to a bearing assembly which is formed of individual bearing surfaces. The bearings are formed of crystalline material and one preferred material is synthetic diamonds, a material believed to be well known and referred to generally hereinafter as PCD. The PCD can be shaped in the form of a circular button. It has an exposed surface. The exposed surface is ideally contacted against an opposing or rotating surface so that the relative motion between the two surfaces engenders continued polishing of the PCD material making up the bearing assembly. In this disclosure, a metal ring is disclosed which serves as a housing or collar on which the buttons are mounted, and the buttons are in turn polished by an opposing surface. Generally, the buttons are described as having a lapped surface which means that they have been polished to provide a very smooth bearing surface. It is possible to make such a bearing assembly by use of techniques to be currently employed. As a generalization, a steel ring is drilled with a number of parallel cylindrical holes. A PCD disk is joined to a cylindrical plug. The plugs are placed in the matching holes in the ring while braze material is placed in the bottom of the hole below the plug. Several such plugs equipped with PCD disks are mounted in like fashion. They have slightly different heights, but differences in height are accomodated by differences in the thickness in the brazed material. Differences in height are avoided as the brazed material is melted. This is normally accomplished by placing the steel ring in a furnace or even between two support plates, one above and one below the ring. The ring is inverted so that the ring is above the individual PCD disks and support plugs on which they are mounted, and all are brought into registration. However, the registration involves, considered vertically from the top to bottom, the disk on the supportive plug, a layer of brazed material of differing thicknesses, and then the steel ring. The layer of brazed material represents a weak area mechanically because it is not as strong as the other materials, and also because the use of the equipment typically encounters chattering and vibration which acts on the brazed metal to cause failure. Therefore the present disclosure sets forth a method of fabrication which has advantages over that procedure, and is more readily implemented in the manufacture of an assembly with the multiplicity of bearing surfaces formed of PCD disks.
This disclosure sets forth a bearing assembly formed in a steel ring which has a number of PCD disk which become lapped in use so that the surfaces are able to support a coacting rotating surface. Heat is liberated in the friction of the two surfaces contacting one another during rotation. Preferably, the equipment is operated in a bath of lubricant material which can in most instances be drilling fluid. The cooling of the disk involves heat transfer from the PCD material into a supporting cylindrical plug and then a supporting steel ring. The steel ring generally transfers heat quite well. In like fashion, the heat transfer characteristics of the supporting plug are normally quite good. The supporting plug can be formed of any number of materials, but in many instances, they are formed of tungsten carbide fabricated into a cylindrical plug. The plug is tungsten carbide in a matrix of support metals which form a plug. The plug is required to transfer heat out of the bearing surface, or restated, transfer heat away from the PCD disk. Heat transfer is a limitation of the device. Thus, very large PCD disks run the risk of damage from excessive heat build up. One approach to reduction of this is to place scored grooves across the face of such disk. This is difficult to achieve because it requires machining the PCD disk with some type of machining device; machining must be carefully controlled, and increases the cost dramatically. The grooves will introduce fluid flow across the face of the PCD disk so that cooling can be obtained.
One advantage of the device of this disclosure is the arrangement of the disks so that they are supported by a cylindrical plug. The plug can be formed in the known fashion of tungsten carbide bits or particles in a supportive alloy matrix which forms the cylindrical plug. The cooperative disk and plug are structurally modified to increase exposure to the circulating fluids about the disk and the supportive plug. The disk is formed of a piece of material which is normally cut in circular fashion and is mounted in the ring so that there is a locus of contact area forming a band around the bearing elements. This defines across the center of the disk a diametric region. It is this region which sustains most of the wear and which provides most of the bearing contact area. In this region, the surface is frictionally engaged and therefore creates most of heat. The portions of the circular disk remote from the diametric region are less significant, and in fact, can be omitted. Accordingly, rather than a circular disk, one version of the present equipment utilizes a simple diametric strip. It can be formed of such a disk which is trimmed so that the remaining portion is a simple strip, ideally having a common width, or having a fixed width in one embodiment. It is not however required that the width of the strip be fixed; if the marginal edges which define the strip are approximately parallel, they need not be straight lines and they need not be parallel; they can be formed of curved segments. They can be formed of many types of curves; they can even be formed of straight line segments. In fact, the strip which provides the diametric support region can be mounted with the support cylinder also broken away or beveled in that region. This increases the proximity of the cooling fluid to the diametric strip which carries the weight.
Going now to the shape of the plug, it can be beveled to reduce its mass immediately adjacent to the diametric support strip. As will be developed, this permits the drilling fluid to circulate closely to the PCD disk. In this construction, the plug and disk are both modified so that they are both exposed to a greater flow of lubricant.
In a modified embodiment, the support plug beneath the PCD disk is fashioned in the shape of an undercut upstanding mounting stem which flares below the PCD disk to support the PCD disk. The upstanding support plug has a nether shoulder which is exposed for fluid flow. This markedly increases the exposed area permitting fluid flow to lubricate and cool the PCD disk and enhances heat transfer from it. In this particular embodiment, the support plug is undercut, defining the nether shoulder or face, and providing a fluid flow channel when multiple similar cooperative support plugs are installed in a single ring.